Methods and apparatus to facilitate ground fault detection with a single coil and an oscillator

ABSTRACT

Methods and apparatus to facilitate ground fault detection with a single coil and an oscillator are disclosed. An example ground fault detection device includes a sense coil including a secondary winding surrounding a line conductor and a neutral conductor, the line conductor and the neutral conductor forming a primary winding. The example ground fault detection device also includes a voltage oscillator connected in series with the secondary winding to drive an oscillation component, and at least one of a detector or an amplifier connected in series with the secondary winding to detect at least one of a neutral-to-ground fault or a line-to-ground fault.

RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 60/864,068 filed Nov. 2, 2006, U.S. provisional application Ser. No.60/864,056 filed Nov. 2, 2006, and U.S. provisional application Ser. No.60/864,058 filed Nov. 2, 2006, all of which are hereby incorporated byreference in their entirety.

FIELD OF THE DISCLOSURE

This disclosure relates generally to ground fault interruption and, moreparticularly, to methods and apparatus to manage ground fault conditionswith a single coil and an oscillator.

BACKGROUND

Electrical power distribution typically includes multiple-conductorwires to transmit electrical energy and facilitate a ground path forsafety. A shock hazard exists in the event of an unintended path fromthe conductor wires or surfaces (such as a chassis of electricalequipment), which carry electric current, and the ground path. Theconductors, such as a line conductor (also referred to as “hot”) and aneutral, or common, conductor, may leak electrical current to eachother, to ground, and/or to a person or object as an intermediate pathto ground. As such, a person in the intermediate path may receive alethal electrical shock.

Ground fault circuit interrupters (GFCIs) may minimize and/or eliminatethe risk of electrical shock by monitoring an imbalance of electricalcurrent between the hot and neutral lines. Generally speaking, aline-to-ground fault may be detected by way of a coil (e.g., a toroidalHall-effect coil) around the line and neutral conductors that provideelectrical energy to a load. Under non-fault operating conditions, themagnetic fields that result from current in the hot conductor cancelwith the magnetic fields that result from an opposite current flow inthe neutral conductor, thereby failing to induce a correspondingelectrical current in the coil. However, if current from the lineconductor leaks current to ground, then the neutral conductor current,and its corresponding magnetic field, will be less than the magneticfield of the line conductor, thereby affecting the coil to produce acorresponding electrical signal indicating a fault. The electrical coilsignal, such as a current value, may be compared to a threshold which,when exceeded, causes the GFCI to force a mechanical break to the loadvia, for example, a circuit interrupter. The circuit interrupter may beemployed as a double pole, single throw switch that, when activated,physically separates the line and the neutral conductors from the load.

Detecting a neutral-to-ground fault poses additional challenges because,in part, the neutral conductor is also grounded at the source. Suchdouble grounding of the neutral conductor could create a situation wherea portion of the fault current from the line conductor returns to thesource through the neutral conductor. As a consequence, the traditionalsingle coil approach will not detect a flux imbalance representative ofthe actual current leakage magnitude. To aid in neutral-to-ground faultdetection, a second coil is typically employed that, when coupled to thefirst coil, produces a positive feedback loop. Despite the lowerdetected current imbalances observed during a neutral-to-ground fault,which may not exceed a tripping threshold, the coupled coils willdevelop an oscillation that, when detected, may be used to indicate acircuit trip or interruption is warranted. Additionally oralternatively, a signal may be injected on the second coil so that, inthe event of a neutral to ground fault, the injected signal is inducedin the neutral line and is detected by the first coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art ground fault circuitinterrupter.

FIG. 2 is a block diagram of an example ground fault detector in whichground faults may be detected with a virtual inductor and a single sensecoil.

FIGS. 3A and 3B are an equivalent circuit diagram and a correspondingoutput plot for the example ground fault detector device of FIG. 2during non-fault conditions.

FIGS. 4A and 4B are an equivalent circuit diagram and a correspondingoutput plot for the example ground fault detector device of FIG. 2during a neutral-to-ground fault.

FIGS. 5A and 5B are an equivalent circuit diagram and a correspondingoutput plot for the example ground fault detector device of FIG. 2during a line-to-ground fault.

FIG. 6 is an example output plot for the example ground fault detectordevice of FIG. 2 during both a neutral-to-ground fault and aline-to-ground fault.

FIG. 7 is a flowchart representative of an example process to implementfault detection in the example ground fault detector device of FIG. 2.

FIG. 8A is a block diagram of another example ground fault detector inwhich ground faults may be detected with a single sense coil.

FIGS. 8B-8D are example equivalent circuits for the example ground faultdetector of FIG. 8A during a non-fault condition, a neutral-to-groundfault, and a line-to-ground fault, respectively.

FIG. 9 is a block diagram of an example ground fault detector to detectfault conditions and simulate a saturation capacitor.

FIG. 10 is a block diagram of an example ground fault detector toperform an automatic self-test.

FIG. 11 includes example output plots for the example ground faultdetector of FIG. 10 during self-test conditions.

FIG. 12 is a flowchart representative of an example process to implementa self-test of the example ground fault detector device of FIG. 10.

FIG. 13 is a block diagram of an example ground fault detector toperform a substantially continuous self-test.

FIGS. 14A and 14B are block diagrams of example fault separationcircuits to facilitate substantially continuous self-test in the exampleground fault detector device of FIG. 13.

FIG. 15 is a flowchart representative of an example process to implementsubstantially continuous self-testing of the example ground faultdetector device of FIG. 13.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a prior art ground fault circuitinterrupter (GFCI) 100. The GFCI 100 electrically connects to a line (or“hot”) conductor 102, a neutral conductor 104, and in some instances, aground conductor 106. During normal operating conditions, the GFCI 100provides electrical power via the line conductor 102 and neutralconductor 104 to an electrical load 108. However, a solenoid relaycontact 110 may break electrical contact with the load 108 if a faultshould occur. For example, faults having a resistive value may occurbetween the line conductor 102 to ground 106 (R_(LG)) or from theneutral conductor 104 to ground 106 (R_(NG)).

As shown in FIG. 1, a first sense coil 112 monitors for a currentimbalance between the line conductor 102 and the neutral conductor 104that may occur when a fault condition between these two conductors ispresent. For example, if no fault condition is present, then all suchcurrent carried by the line conductor 102 is also carried by the neutralconductor 104, but in the opposite direction. When a fault occurs, someof the current provided by the line conductor 102 takes an alternatepath that bypasses the first sense coil 112, thereby causing n current(I₁) induced to a secondary winding 114 of the first sense coil 112.Generally speaking, the sense coil may be employed as a toroidal coilconstructed with high permeability laminated steel rings. A primary ofthe sense coil may be a single turn, through which the line and neutralconductors pass, while a secondary winding of the sense coil mayinclude, for example, approximately 200 to 1500 turns. Current valuesdetected by the first coil 112 are received by a ground fault detectorcircuit 116 powered by a bridge circuit 117, and such received signalsare compared against a threshold, which is typically established in viewof regulatory standards (e.g., Underwriters Laboratories). Theestablished threshold(s) may pertain to a maximum current threshold(e.g., trip if the detected current exceeds a root mean square value of6 milliamps) and/or a maximum duration of observed current imbalance(e.g., trip if detected current is sustained for more than threeline-cycles). The ground fault detector circuit 116 responds to anexceeded threshold by sending one or more signals to a switching device118, such as a silicon controlled rectifier (SCR), which may drive thesolenoid relay contacts 110 to break the electrical contact with theload 108.

In the event of a neutral to ground fault, the first sense coil 112 maynot sense any current imbalance, thus fail to trip the solenoid relaycontact(s) 110. To detect a neutral to ground fault (R_(NG)), a secondcoil 120 is employed having a secondary coil winding 122 to which theground fault detector circuit 116 applies an oscillation signal. Duringnon-fault operating conditions, the signal is not induced into theconductors because there is no impedance imbalance between the lineconductor 102 and the neutral conductor 104 (no path between neutral andground). However, in the event of a ground to neutral fault, the appliedoscillation signal from the second coil 120 will propagate to theneutral-to-ground path and is then sensed by the first sense coil 112.When the corresponding oscillation signal is detected by the first sensecoil 112, and processed by the ground fault detector circuit 116, theswitching device may cause the relay contact 110 to break electricalcontact.

The GFCI 100 also includes a test switch 124 and fault resistor R_(F) toallow an end user (e.g., a homeowner) to verify that the GFCI 100 isworking properly. When the test switch 124 is activated, current fromthe line conductor 102 passes through the first sense coil 112 in afirst direction, but is diverted through R_(F) and the test switch 124to the neutral conductor 104. As a result, the returning current (in asecond/opposite direction) on the neutral conductor 104 as it passesthrough the first sense coil 112 is less than the current on the lineconductor 102 when it passes through the first sense coil 112, whichcauses a detected current imbalance between the line conductor 102 andthe neutral conductor 104. However, a manual test process with the testswitch 124 may not be a regular practice of the end user, which may meanthat the end user is not protected in the event of a fault that may haveoccurred in between manual tests, if any.

The GFCI 100 also includes a saturation capacitor (C_(S)) for the firstsense coil 112 and the second coil 120 to, in part, prevent and/orminimize saturation of one or more voltage-to-current amplifiers thatare connected to the coils. For example, an amplifier and/or avoltage-to-current amplifier/converter typically has a very low inputimpedance when configured with negative feedback. The secondary sensecoil winding 114 behaves as an inductor and operates similar to a shortcircuit at low frequencies. Such operation creates a very high gainconfiguration, thus small direct current (DC) offsets that are typicallypresent in operational amplifiers (OPAMPS) and/or similar devices maysaturate the output. The output saturation may be eliminated with C_(S),which operates as a DC block yet allows AC signals to pass therethrough.Capacitors must typically have relatively large values to both block DCand pass household line frequency signals (e.g., 50 Hz, 60 Hz), whichcorrespond to added manufacturing costs for the GFCI 100. Additionally,added cost results from having multiple coils within the GFCI 100.

The various methods and apparatus described herein may facilitate groundfault detection without a second coil, thereby eliminating an additionalpotential point of failure and enabling a cost savings opportunity thatmay be passed on to end users. Furthermore, the methods and apparatusdescribed herein may facilitate sense coil operation with a smallercapacitor, thereby saving additional costs related to manufacture of theground fault detection device. While self test techniques may still beperformed by the end user, the apparatus and methods described hereinmay facilitate automatic self testing on a periodic and/or continuousbasis to allow identification of unsafe conditions before an end user isexposed to potential danger due to, for example, failure of one or moreparts of a ground fault detection device to operate correctly. Some orall of the methods and apparatus described above may be used with oneanother.

Coil Elimination

Turning to FIG. 2, a portion of a ground fault detector device 200 isshown having a ground fault detector circuit 202. The example groundfault detector circuit 202 (GFDC) may operate instead of, for example,the ground fault detector circuit 116 and corresponding second coil 120of FIG. 1. The ground fault detector circuit 202 receives signals from asense coil 204 having a primary winding through which a line conductor206 and a neutral conductor 208 pass. As described in further detailbelow, the sense coil 204 allows the GFDC 202 to detect a currentimbalance between the line conductor 206 and the neutral conductor 208during fault conditions. Moreover, the GFDC 202 may detect a faultbetween the line conductor 206 and a ground 210, and/or the neutralconductor 208 and the ground 210.

The sense coil 204 includes a secondary winding (L₁), which is connectedin parallel with a capacitor (C₁) and an active inductor (L₂) that issimulated by voltage to current converters G_(M2), G_(M3), and acapacitor C₂. Persons having ordinary skill in the art will appreciatethat the voltage to current converters G_(M2), G_(M3), and the capacitorC₂ may be referred to as a virtual inductor 214. The value of L₂ isdesigned to be much smaller than L₁, thus the overall inductance valueof the parallel combination of L₂ and L₁ has a value of, for allpractical purposes, L₂. The parallel combination of the inductors (L₁and L₂) and C₁ form a resonance circuit, such as an LC tank that is alsoin parallel with a negative resistance (formed by transconductanceG_(M1) in positive feedback) 212, which operates as a driving source togenerate oscillations. The amplitude of oscillations may be limited witha limiter 215, as shown in FIG. 2. The limiter 215 may include, but isnot limited to, a pair of diodes, zener diodes, etc. In the illustratedGFDC of FIG. 2, the virtual inductor L₂ 214 may perform a dual role thata traditional inductor may not facilitate. In particular, the virtualinductor L₂ 214 allows oscillations by virtue of the LC tank and, unlikea traditional inductor, also allows the measurement of the current goingthrough. Since L2 is much smaller than L1, almost the totality of thecurrent induced into the secondary winding 114 caused by a neutral toground fault, can be measured.

A saturation capacitor C_(S) is shown in series with L₁ to minimizeand/or eliminate amplifier saturation during DC conditions. However,C_(S) may be reduced as a cost savings initiative while preservingsaturation prevention, as described in further detail below. Personshaving ordinary skill in the art will appreciate that voltage-to-currentconverters may be rated by a transconductance value g_(M). Thetransconductance is a ratio of the output current and an input voltage.Additionally, negative resistors may be implemented by, for example, oneor more voltage-to-current amplifiers. A low pass filter 216 andneutral-to-ground (NG) detector 218 are connected to the output of thevirtual inductor 214, and a switch driver 220 is connected to the NGdetector 218 and a line-to-ground (LG) detector 222, discussed infurther detail below. The example switch driver 220 may include a gatedriver control for solid state switching devices such as, for example,bi-polar junction transistors (BJTs), metal oxide field effecttransistors (MOSFETs), silicon controlled rectifiers (SCRs), etc.

If no fault condition is present (i.e., an NG and/or an LG fault), thenL₁, C₁, −R₁ (or G_(M1)), and the virtual inductor 216 operate as anon-fault equivalent circuit 302 shown in FIG. 3A. In the illustratednon-fault equivalent circuit 302 of FIG. 3A, the voltage at node V₁ isproportional to the current I_(L) passing through the virtual inductorL₂. Voltage V₁ may be represented as shown below in Equation 1.

$\begin{matrix}{V_{1} = \frac{I_{L}}{G_{M\; 3}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

The equivalent circuit of FIG. 3A behaves as an oscillator to produce anoscillating current I_(L), having a resonance frequency (f) asrepresented by Equation 2. The resonance frequency may be detected bythe NG detector 218.

$\begin{matrix}{f = \frac{1}{2\pi\; L_{2}C}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

The NG detector 218 may be implemented in one or more ways including,but not limited to, a full wave rectifier connected to a comparator, adigital filter, etc. However, the resonance frequency component at nodeV₁ is blocked by the low pass filter 216 so that, under non-faultconditions, node V₁ includes a resonance frequency component while nodeV₂ does not include the resonance frequency component, as shown in FIG.3B. The LG detector 222 may be employed to confirm that no faults existby, for example, verifying the magnitude of signal at node V₂ is smallenough.

If a neutral-to-ground (NG) fault is present, then the example GFDC 202may be electrically represented by a neutral-to-ground (NG) equivalentcircuit 402 shown in FIG. 4A. In the illustrated NG equivalent circuit402 of FIG. 4A, resistor R₂ represents the NG resistance, which disruptsthe oscillation previously established by the combination of L₁, C₁,−R₁, and L₂. The value of R₂ is shown below in Equation 3.R ₂ =n ² *R _(G)  (Equation 3)

In the example Equation 3, n is the number of turns of the coil L₁, andR_(G) is the NG resistance. When R₂ is less than the negative resistanceR₁, the overall resistive value becomes positive and there is no longera condition for oscillation and the resonance frequency component islost. As a result, the NG detector 218 will not detect oscillations atnode V₁, as shown in FIG. 4B, and, therefore, will trigger the switchdriver 220.

If a line-to-ground fault (LG) is present, then the example GFDC 202 maybe electrically represented by an LG equivalent circuit 502 as shown inFIG. 5A. In the illustrated LG equivalent circuit 502, current I₁represents a line current fault (which is a function of the imbalancecurrent divided by the number of turns n) directly to ground at a sourcefrequency (e.g., a U.S. domestic frequency of 60 Hertz (Hz)). Duringsuch an LG fault, node V₁ may still exhibit a signal component atresonance frequency by virtue of the LC tank (i.e., L₂ and C₁) driven bythe negative resistance (−R₁). As such, detection of the properties ofthe signal at the NG detector 218 may not be sufficient to ascertainthat a fault has occurred. However, the low pass filter 216 maysegregate the relatively higher frequency component leaving lowerfrequency components (e.g., 60 Hz) at node V₂, as shown in FIG. 5B. Themagnitude of the signal at line frequency (e.g., 60 Hz), detected by theLG detector 222 may cause the LG detector 222 to signal the switchdriver 220, thereby allowing the example GFDC 202 to protect an end-userfrom a potential shock hazard. Additionally or alternatively, the switchdriver may provide a fault indication to a user by driving an indicatorlamp, a light-emitting diode (LED), and/or other indicia of a faultcondition.

If both an NG fault and an LG fault are simultaneously present, theexample GFDC 202 may detect such a condition and signal the switchdriver 220, when appropriate. In the illustrated example node output ofFIG. 6, the NG detector 218 detects an NG fault by virtue of the signalcomponent at resonance frequency being absent at node V₁. Additionally,the LG detector 222 detects an LG fault by virtue of the properties ofthe signal at node V₂ (e.g., an amplitude, a duration, etc.)

FIG. 7 illustrates a flowchart representative of an example process thatmay be executed to implement the example GFDC 202 illustrated in FIG. 2.The example process of FIG. 7, as well as the example processes of FIGS.12 and 15 discussed in further detail below, may be implemented usingmachine readable instructions executed by a processor, a controller,and/or any other suitable processing device. For example, the exampleprocesses of FIGS. 7, 12, and 15 may be embodied in coded instructionsstored on a tangible medium such as a flash memory, or RAM associatedwith a processor (e.g., a controller, a microprocessor, a DSP).Alternatively, some or all of the example processes shown in theflowcharts of FIGS. 7, 12, and 15 may be implemented using anapplication specific integrated circuit (ASIC), a programmable logicdevice (PLD), a field programmable logic device (FPLD), discrete logic,hardware, firmware, etc. Also, some or all of the example processes ofFIGS. 7, 12, and 15 may be implemented manually or as combinations ofany of the foregoing techniques, for example, a combination of firmwareand/or software and hardware may be implemented using circuits based onpassive or active circuit components such as resistors, capacitors,inductors, operational amplifiers, etc.

Further, although the example processes of FIGS. 7, 12, and 15 aredescribed with reference to the flowcharts of FIGS. 7, 12, and 15,persons having ordinary skill in the art will readily appreciate thatmany other methods of implementing the example GFDC 202, the exampleGFDC 802, the example GFDC 902, the example ground fault interruptor1000, the example GFDC 1300, and/or the example circuits 1400 and 1450,respectively, illustrated in FIGS. 2, 8A, 9, 10, 13, 14A and 14B may beemployed. For example, the order of execution of the blocks may bechanged, and/or some of the blocks described may be changed, eliminated,sub-divided, or combined. Additionally, persons of ordinary skill in theart will appreciate that the example processes of FIGS. 7, 12, and 15 becarried out sequentially and/or carried out in parallel by, for example,separate processing threads, processors, devices, circuits, etc.

The example process 700 of FIG. 7 begins with, for example, the NGdetector 218 monitoring node V₁ for the presence of a signal (block702). If the NG detector 218 does not detect a signal at the resonancefrequency, the example LG detector 222 may determine whether the outputsignal from the low pass filter 216 is within appropriate thresholds(block 710). The example NG detector 218 may then trigger the gatedriver 220 as a condition indicative of an NG fault (block 704). On theother hand, if the NG detector 218 detects a signal at node V₁, NGdetector 218 may determine whether the detected signal is withinacceptable threshold boundaries (block 706). For example, temperaturefluctuations may result in oscillation frequency variation of theexample virtual inductor 214 that are deemed to be within acceptablelimits. Additionally or alternatively, the NG detector 218 may determineif the properties of the signal are within acceptable limits, andtrigger the gate driver 220 if not (block 704).

Even if the NG detector 218 detects a signal at node V₁ (block 702), andthe signal is within acceptable threshold limits (block 706), the LGdetector 222 may monitor for a signal after the low pass filter 216 atnode V₂ (block 708). As described above, the example low pass filter 216attenuates the oscillation frequency component caused by the parallelcombination of L₁, C₁, −R₁, and the inductor L₂ (e.g., the virtualinductor 214). In the absence of the oscillation frequency, the LGdetector 222 may determine whether a relatively lower frequency ispresent, such as a frequency at or near the source of a household powerline (e.g., 50 Hz, 60 Hz, etc.) (block 710). Without limitation, theexample LG detector 222 may determine whether the output signal from thelow pass filter 216 is within appropriate thresholds (e.g., magnitude,duration, etc.) (block 710). If not, the LG detector 222 signals thegate driver 220 to trip so that the example GFDC 202 stops providingelectrical energy to the load (block 712). However, if the LG detector222 determines that the signal properties are within the establishedlimits (block 710), then no fault has occurred and the example process700 continues to monitor nodes V₁ and V₂ for both NG and LG faults.

FIG. 8A illustrates an additional example GFDC 802 to detect LG and NGfaults with one coil rather than two. In the illustrated example of FIG.8, the GFDC 802 may operate instead of the ground fault detector circuit116 and corresponding second coil 120 of FIG. 1. The GFDC 802 receivessignals from a sense coil 804 having a primary winding through which aline conductor 806 and a neutral conductor 808 pass. Current imbalancesbetween the line conductor 806 and the neutral conductor 808 may bedetected by the example sense coil 804 during fault conditions, in whichfault conditions include an LG fault and/or an NG fault.

The sense coil 804 includes a secondary winding L₁ that is connected toan oscillator S₁, and a current-to-voltage amplifier 810 via asaturation capacitor C_(S) prevent and/or minimize amplifier saturationduring DC conditions. However, the example GFDC 802 of FIG. 8A mayemploy methods and apparatus to eliminate C_(S) while preservingsaturation prevention functionality, as discussed in further detailbelow. The example GFDC 802 also includes a low pass filter 812, an NGdetector 814, and an LG detector 816. Both the NG detector 814 and theLG detector 816 may provide one or more signals to a driver 818 in theevent of a fault. Persons having ordinary skill in the art willappreciate that the driver 818 may include, but is not limited to, anSCR driver, a transistor driver, a relay driver, etc.

If there are no faults (either LG and/or NG), then a correspondingnon-fault equivalent circuit 850 results, as shown in FIG. 8B. Generallyspeaking, based on the secondary winding L₁, which is the magnetizationinductance of the sense coil 804, and the frequency of the oscillatorS₁, the current into the current to voltage amplifier 810 may berepresented by Equation 4.

$\begin{matrix}{I = \frac{V_{OSC}}{L_{1}*2\;\pi\; f}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

In example Equation 4 above, the value V_(OSC) represents the voltage ofoscillator S₁, L₁ is the magnetization inductance of the sense coil 804,and f is the applied frequency of the oscillator S₁. Absent any faultcondition, the NG detector 814 may determine that the current is withinone or more appropriate thresholds. However, in the event of a NG fault,a corresponding NG fault equivalent circuit 852 results, as shown inFIG. 8C. In the example NG fault equivalent circuit 852 of FIG. 8C, R₂represents an equivalent resistance based on a NG resistance R_(G) and anumber of turns (n) in the sense coil 804, mathematically represented inexample Equation 5.R ₂ =n ² *R _(G)  (Equation 5)

As a result, the current detected by the current to voltage amplifier810 is mathematically represented in example Equation 6.

$\begin{matrix}{I \approx {\frac{V_{OSC}}{R_{G}}\mspace{14mu}{equation}\mspace{14mu}{has}\mspace{14mu}{changed}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

The example NG detector 814 may determine that, as a result of the addedresistance R_(G) in parallel with the inductor L1, the amplitude of thesignal increases and one or more thresholds have been exceeded, therebyindicating a fault condition. One or more signals may be sent by the NGdetector 814 to the driver 818 to activate one or more safety devices,such as a relay to break the line-conductor 806 and/or neutral conductor808 from an electrical source.

In the event of an LG fault, an LG fault equivalent circuit 854 results,as shown in FIG. 8D. In the example LG fault equivalent circuit 854 ofFIG. 8D, I₂ represents additional current flow detected as a result ofdirect contact between the line-conductor 806 and a ground conductor.However, because this current (I₂) is an additional imbalance currenthaving a frequency equal to that of the power line source (e.g., a U.S.domestic 60 Hz line-power), the example low pass filter 812 mayattenuate the relatively higher oscillation frequency component andreveal the fault line frequency component. The example LG detector 816may sense this signal (e.g., at 60 Hz) and, in response, send one ormore trip signals to the driver 818.

Saturation Capacitor Reduction

As described above in connection with FIG. 1, the saturation capacitorC_(S) connected to the first sense coil 112 and the second coil 120allow for detection of current imbalance conditions without causing asaturation of the amplifier during DC input conditions. While thesaturation capacitors C_(S) of FIG. 1 permit AC signals to pass andblock the DC signals, the size of such saturation capacitors C_(S) isdirectly proportional to the cost of the GFCI 100. FIG. 9 illustrates aportion of a ground fault detector device 900, which includes a groundfault detector circuit (GFDC) 902. The example GFDC 902 may operateinstead of, for example, the ground fault detector circuit 116 andsaturation capacitor(s) C_(S) of FIG. 1, and includes a firsttransconductor architecture sub circuit 904, a second transconductorarchitecture sub circuit 906, a detector 908, and a switch driver 910 toelectrically trigger one or more safety devices (e.g., a mechanicalrelay to break electrical contact to an electrical source).

The first transconductor architecture sub circuit 904 includes avoltage-to-current converter G_(M1) (having a correspondingtransconductance g_(M)) having a positive input 912 to which a sensecoil 914 is connected. A line conductor 916 and a neutral conductor 918pass through the sense coil 914 to monitor for, in part, currentimbalances therebetween. However, the sense coil 914 in the illustratedexample of FIG. 9 does not include a saturation capacitor that typicallyaccompanies prior art sense coils, thereby allowing for cost reduction.

In operation, the voltage-to-current converter G_(M1) current outputnode 920 is connected to a positive input node 922 of voltage-to-currentconverter G_(M2), in which both of the voltage-to-current converters(G_(M1) and G_(M2)) are within the first transconductor architecture subcircuit 904. Additionally, the second transconductor architecture subcircuit 906 includes a voltage-to-current converter G_(M3) having apositive input node 924, which is electrically connected to the currentoutput node 920 of G_(M). Much like the first transconductorarchitecture sub circuit 904, the second transconductor architecture subcircuit 906 includes a second voltage-to-current converter (G_(M4))having a positive input node 926 and a current input node 928. However,rather than incorporate a saturation capacitor C_(S), as is shown inFIG. 1, the second transconductor architecture sub circuit 906 of FIG. 9includes capacitor C₃ connected at a current output node 930 of G_(M3)and the positive input node 926 of G_(M4), and a negative rail 932.Capacitor C₃ is smaller than capacitive values typically required forsaturation purposes and is, thus, more cost effective when incorporatedwithin the transconductor architecture sub circuit 906.

Each of the voltage-to-current converters G_(M1), G_(M2), G_(M3), andG_(M4) have corresponding transconductance values of g_(M1), g_(M2),g_(M3), and g_(M4). The effect of the first and second transconductorarchitecture sub circuits 904 and 906 results in an equivalentsaturation capacitance value as shown by Equation 7. Therefore, C₃ maybe much smaller than C_(S).

$\begin{matrix}{C_{S} = \frac{C_{3}g_{M\; 1}g_{M\; 2}}{g_{M\; 3}g_{M\; 4}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

The first and second transconductor architecture sub circuits 904 and906 in conjunction with C₃, prevent output saturation of thevoltage-to-current converters, and the signal at node 934 that containsthe information of the current is fed to the detector 908. Withoutlimitation, the example detector 908 of FIG. 9 may include the GFDC 802of FIG. 8A or the example GFDC 202 of FIG. 2.

Automatic GFCI Testing

As described above in connection with FIG. 1, prior art GFCIs 100include a test switch 124 and a fault resistor R_(F) that allow currentfrom the line conductor 102 to bypass a return path through the sensecoil 112. As a result, the sense coil 112 will sense the incomingcurrent from the line conductor 102, but not any returning currentthrough the neutral conductor 104, thereby causing a current signal tobe induced on the sense coil 112. Such a current imbalance signal, whendetected by the ground fault detector circuit 116, causes the siliconcontrolled rectifier (SCR) 118 to turn on, which energizes a relay coilin the solenoid relay contact 110. The energized relay coil in thesolenoid relay contact 110 breaks electrical contact from the sourceenergy supplied by the line conductor 102 and the neutral conductor 104,thereby preventing and/or minimizing the potential for an electricshock.

While the test switch 124 allows the GFCI 100 to be operationally testedby simulating a fault condition, such verification of proper GFCIfunctionality and/or capabilities is dependent upon regular testing bythe end user. In the event the end user fails to test the prior art GFCI100 on a periodic and/or regular basis, one or more fault conditions mayarise in between tests. Furthermore, if the GFCI 100 fails to operatecorrectly due to, for example, a failed sense coil, a failed saturationcapacitor C_(S), etc., then the end user will not be protected in case afault occurs. In such circumstances, merely having a GFCI present mayprovide a false sense of security to the end user.

FIG. 10 illustrates an example ground fault detector device 1000electrically connected to a hot (line) conductor 1002 and a neutralconductor 1004. An electrical and/or earth ground 1006 may be located atan electrical source 1008, such as at an electrical panel located withinan end user's household. Persons having ordinary skill in the art willappreciate that the ground 1006 at the electrical source 1008 istypically connected to the neutral conductor 1004. However, the ground1006 from the electrical source 1008 is typically distributed throughoutthe household with no other intentional contact with either the neutralconductor 1004 and/or the hot conductor 1002. In other words, anycontact between ground 1006 and the neutral conductor 1004 in a locationother than the source 1008 is referred to as a neutral to ground fault.

The example ground fault detector device 1000 includes a power diodebank 1010, a fault diode bank 1012, a relay 1014 to break electricalpower with the source 1008 in the event of a fault, and a relay coil1016 that, when energized, causes the relay coil 1016 to open. A switch1018 is electrically connected to the relay coil 1016, which may beimplemented as an SCR or a transistor (e.g., a metal oxide semiconductorfield effect transistor (MOSFET)). The switch 1018 receives one or moresignals from a ground fault detector circuit (GFDC) 1020 based on, forexample, fault condition information and/or self-test request(s). In theillustrated example of FIG. 10, the switch 1018 is implemented as aMOSFET and receives one or more control signals from the GFDC 1020 via agate pin 1022. As described in further detail below, because a MOSFETmay be driven to create one or more channel sizes, the MOSFET may enableone or more levels of conduction of the relay coil 1016. In other words,the example MOSFET 1018 of FIG. 10 may have a full-on state, in whichthe relay coil opens, or one or more partially-on states, in whichcurrent may flow through the relay coil 1016 without opening the relay1014 to break electrical power with the source 1008. Additionally oralternatively, the MOSFET 1018 and/or other switch may be implementedwithin the example ground fault detector device 1000 independently ofthe relay coil 1016.

The example GFDC 1020 of FIG. 10 includes a switch driver 1024 that,when signaled by a switch logic module 1026 and/or a self-test logicmodule 1028, elicits a signal to the gate pin 1022. A watchdog timer1030 is operatively connected to the self-test logic module 1028 toprevent a runaway test condition. Persons having ordinary skill in theart will appreciate that the watchdog timer 1030 may, additionally oralternatively, be embodied in a controller 1032. One or morevoltage-to-current converters and/or one or more differential amplifiers1034 may receive one or more signals from a sense coil 1036. While theexample ground fault detector device 1000 illustrates a saturationcapacitor C_(S) connected to the sense coil 1036, the example first andsecond transconductor architecture sub-circuits (904 and 906) may beemployed to eliminate C_(S) and, instead, use a smaller capacitor. Thedifferential amplifiers 1034 may employ the GFDC 202 of FIG. 2, the GFDC802 of FIG. 8A, and/or include the first and second transconductorarchitecture sub circuits 904 and 906 of FIG. 9. The output of theexample differential amplifiers 1034 may be applied to one or morefilters 1038 and/or the gate logic module 1026 to determine one or morecontrol function(s) (e.g., turn MOSFET 1018 partially-on for self test,turn MOSFET 1018 completely on due to a fault, turn MOSFET 1018 off,etc.).

Without limitation, the example GFDC 1020 may include one or moreadditional or alternative modules for other purposes, such as an examplezero-cross module 1040. For example, in the event that the switch 1018is employed as an SCR, then the zero-cross module 1040 may signal thecontroller 1032 when the voltage phase of the source waveform is at azero-cross point. Such information may be useful to determine when toturn-on the SCR, or such information may be useful to count how manysource power cycles elapse during a suspected fault condition. If morethan three power cycles elapse (i.e., 6 zero-cross detections) while,for example, the sense coil 1036 exhibits a current imbalance, then thecontroller 1032 may signal the switch driver 1024 to energize the switch1018 via the gate pin 1022.

In operation, the example power diode bank 1010 provides power to theGFDC 1020 via four diodes D₁, D₂, D₃, and D₄. The four diodes form afull wave bridge to supply a positive voltage to the GFDC 1020 so thatwhen current flows into diodes D₁ and D₂ (e.g., during a positive powercycle), no current flows at the hot conductor 1002 and the neutralconductor 1004 passing through the sense coil 1036, thereby preventingany current imbalance that may cause a fault trip. Diode pairs D₁ and D₂may form a power tap for one half-phase of a power-line cycle (e.g., apositive ½ phase of a 60 Hz domestic household), and diode pairs D₃ andD₄ may form another power tap for the second half-phase of thepower-line cycle. As shown in FIG. 10, the first power tap (i.e., diodepair of D₁ and D₂) is at a location prior to the sense coil 1036.Additionally, the second power tap (i.e., diode pair D₃ and D₄) is at alocation after the example sense coil 1036. When current flows intodiodes D₃ and D₄ (e.g., during a negative power cycle), the currentpassing through the coil on the hot conductor 1002 and the neutralconductor 1004 are the same, thereby preventing the sense coil 1036 fromdetecting any current imbalance. Although the illustrated example ofFIG. 10 illustrates diodes D1, D3, and D5 connected to the neutralconductor 1006, and diodes D2, D4, and D6 connected to the lineconductor 1002, such orientation is not limited thereto. For example,the first power tap (e.g., diodes D1 and D2) may be connected to theline conductor 1002 and the line conductor 1004, respectively, while thesecond power tap (e.g., diodes D3 and D4) may be connected to the lineconductor 1002 and the and the neutral conductor 1004, respectively.Additionally, while the illustrated example of FIG. 10 includesfull-wave capabilities, the example switch 1018 may also operate toperform self test and signal the relay 1014 during half-wave operation.

At scheduled and/or periodic times, the example self-test logic module1028 is invoked (e.g., invoked by the controller 1032) to partiallyturn-on the MOSFET 1018 via the switch driver 1024 (e.g., a MOSFET gatedriver) and the gate pin 1022. As a result, diodes D₅ and D₆ of thefault diode bank 1012 may conduct during alternate power phases. DiodeD₅ establishes a first fault tap as a fault diode at a first locationprior to the sense coil 1036, and diode D₆ establishes a second faulttap as a fault diode at a second location after the sense coil 1036. Forexample, during a positive power cycle, D₆ conducts and current flowsfrom the hot conductor 1002 after passing through the sense coil 1036 asa current source path. The current continues to flow through the relaycoil 1016, but because the switch driver 1024 is not fully turned-on,insufficient current passes through the relay coil 1016 to cause therelay 1014 to open. The current continues to flow through the MOSFET1018 and a fault resistor R_(F) to ground and through D₁ to the neutralconductor 1004 as a current bypass path. However, because the currentreturning to the neutral conductor is located at a point preceding(i.e., the first power tap) the sense coil 1036, a fault is detected.Rather than trip the relay coil 1016, the controller 1032 and/or thegate logic module 1026 may associate the fault with the active testconditions so that the relay coil is not opened unnecessarily.

Similarly, during a negative power cycle, D₅ conducts and current flowsfrom the neutral conductor 1004 before passing through the sense coil1036 as a current source path. The current flows through the relay coil1016 and, because the switch driver 1024 is not fully turned-on,insufficient current passes through the relay coil 1016 to cause therelay 1014 to open. The negative power cycle current continues to flowthrough the MOSFET 1018 and the fault resistor R_(F) to ground andthrough D₄ to the line conductor 1002 as a current bypass path. In viewof the fact that the current returning to the line conductor 1002 islocated at a point after (i.e., a second power tap) the sense coil 1036,a fault is detected. As described above, the relay coil 1016 is nottripped despite sensing this fault because the controller 1032 and/orthe gate logic module 1026 expects the fault during the duration of theself-test.

Additionally or alternatively, the example GFDC 1020 may include anoperational amplifier (OPAMP) or comparator 1042 to compare a potentialof the MOSFET 1018 output against a predetermined setpoint 1044. Anoutput of the OPAMP 1042 may allow the self-test logic module 1028 toadjust how hard or how softly the switch driver 1024 drives (conducts)the MOSFET 1018. As a result, one or more test patterns may be appliedto the MOSFET 1018 (or any other type of switch) to, for example,evaluate both real and imaginary components of the switch impedancewithout unnecessarily engaging the relay coil 1016. Additionally,driving the MOSFET 1018 at one or more alternate levels of conductionmay allow regulation and testing of the electro-mechanical integrity ofthe relay coil 1016 (e.g., to determine whether the relay coil 1016opens and/or closes within acceptable electrical specifications,temperature specifications, noise conditions, time-to-close, etc.).Without limitation, one or more switch 1018 tests may include a shortcircuit test, an inductance test, and/or a response-time test.

FIG. 11 illustrates example waveforms for the ground fault detectordevice 1000 shown in FIG. 10. In the illustrated example of FIG. 11, aline voltage waveform 1102 indicates phases of the power cycles exitedby the source 1008, and a post-bridge waveform 1104 indicatescorresponding phases of the power cycles after the power diode bank 1010(i.e., diodes D₁, D₂, D₃, and D₄). At time t₁, the example self-testlogic module 1028 or the controller 1032 partially turns-on the switch1018 (e.g., a MOSFET, a bipolar junction transistor, etc.), and atransistor current waveform 1106 indicates the current conducted as aresult of switch 1018 activation. As described above, because switch1018 activation allows the diodes in the fault diode bank 1012 toconduct, a full wave fault is simulated with the same signal propertiesas a real fault. The full wave fault is detected by the sense coil 1036,as shown by a detected fault current waveform 1108. The example groundfault detector device 1000 may perform the test for a predeterminednumber of power cycles, as determined by, for example, the zero-crossmodule 1040. However, the self-test may stop at time t₂ based on, butnot limited to, a timer expiration in the example controller 1032, thewatchdog 1030, and/or the example self-test logic module 1028.

FIG. 12 illustrates a flowchart representative of an example process1200 that may be executed to implement the example ground fault detectordevice 1000 and corresponding self-test of FIG. 10. The example process1200 of FIG. 12 begins with, for example, the controller 1032 and/or theself-test logic module 1028 initiating a self-test (block 1202). Personshaving ordinary skill in the art will appreciate that the controller1032 may include an on-board timer and/or logic that employs one or moreregisters to determine a time-base. For example, the controller maymonitor the zero-cross module 1040 for a predetermined number of powerline cycles before initiating the self test.

If the example ground fault detector device 1000 is not executing aself-test (block 1202), then the voltage-to-current converter 1034monitors the sense coil 1036 for a current imbalance (block 1204). If nocurrent imbalance is detected by the voltage-to-current converter 1034,then the example GFDC 1020 continues to monitor for fault conditions orinitiates a self-test mode (block 1202). However, if a current imbalanceis detected (block 1204), then the switch logic module 1026 and/or thefilters 1038 determine if one or more thresholds are exceeded (block1206). For example, power line transients may occur periodically and/orintermittently that result in some current and/or frequency fluctuationsdetected by the sense coil 1036. As a result of some expectedfluctuation (s), the filters 1038 and/or gate logic module 1026 mayemploy one or more thresholds so that the relay 1014 is not tripped intoan open state unnecessarily.

If the one or more signal properties (e.g., an amplitude, a duration, anumber of cycles, a frequency, etc.) are not exceeded, then the exampleprocess 1200 returns to block 1202 to continue monitoring for faultconditions and self-test mode. In the event that the signal propertiesare outside one or more limits (e.g., outside one or more threshold(s)of the one or more signal properties) such as, for example, a detectedcurrent value exceeding 0.01 mA for more than three power cycles, thenthe switch logic module 1026 provides a signal to the switch driver 1024to energize the switch 1018 (block 1208). Thresholds may also include,but are not limited to, a current level that, when detected, cause therelay coil 1014 to open regardless of how many power line cycles elapseduring the detected current level.

If a self test mode initiation is invoked by, for example, apredetermined number of zero-cross cycles or an expiration of a timer onthe controller 1032 and/or the self-test logic module 1028 (block 1202),then the self-test logic module 1028 provides a signal to the switchdriver 1024 to partially turn on the switch 1018 (block 1210). As aresult, the switch allows current to flow through the fault diode bank1012 (i.e., diodes D₅ and D₆) and the relay coil 1016. As describedabove, the self-test logic module 1028 may regulate how much the switch1018 conducts so that current may flow through the relay coil 1016without opening the relay 1014. Assuming, for example, that diode D₆conducts during the positive ½ power cycle, current flowing through theswitch 1018 causes a voltage drop across R_(F), which continues to theneutral conductor 1004 via D₁. The current that conducted through D₆from the hot conductor 1002 passed through the sense coil 1036, howeverthe return current path to the neutral conductor 1004 via D₁ bypassedthe sense coil 1036, thereby resulting in a simulated fault. If theexample ground fault detector device 1000 and/or the GFDC 1020 isworking properly, the voltage-to-current converter 1034 will detect thisimbalance as a fault (block 1212). To prevent unnecessarily opening therelay 1014 during self-test, the controller 1032 and/or the self-testlogic module 1028 may disable the gate drive 1024 when a known self testis executing. However, if the self test is executing and no currentfluctuation is detected, then the controller 1032 and/or the self-testlogic module 1028 causes the switch 1018 to fully energize so that therelay 1014 opens (block 1208).

A self test initiated by the controller 1032 and/or the self-test logicmodule 1028 may be limited in duration to minimize instances where theexample ground fault detector device 1000 may not be protecting againstan actual fault. For example, the controller 1032 and/or the self-testlogic module 1028 may employ the zero-cross detector 1040 to monitor fora predetermined number of power line cycles for which the self-testexecutes. Upon expiration of the predetermined number of power linecycles, the self-test stops executing and fault detection resumes. Inthe event that an actual fault occurs at a moment that overlaps with theself-test, then actual exposure to potential shock hazard(s) may beminimized and/or avoided by minimizing the duration of the self-test. Inother words, if the self test includes a power line threshold limitationof three power cycles, then an end user of the example ground faultdetector device 1000 will not be exposed to potential shock conditionsfor more than approximately 50 milliseconds. Persons having ordinaryskill in the art will appreciate that the threshold number of power linecycles and/or a threshold time limit may be set and/or adjustedaccording to other factors including, but not limited to, regulation(s)imposed by Underwriters Laboratories.

The example GFDC 1020 may also determine whether a fault signal wasdetected by the OPAMP 1042 (block 1214). Failure to sense a fault signalwithin one or more thresholds established by the setpoint 1044 (block1216) may be indicative of one or more circuit failures at one or morecomponents of the example ground fault detector device 1000. As a resultof not detecting, for example, a voltage at a MOSFET source 1046 duringa known self test, the controller 1032 and/or the self-test logic module1028 may cause the switch to fully energize, thereby causing the relay1014 to open (block 1208) and/or display a warning/failure light toalert the user. On the other hand, if the OPAMP 1042 detects the faultsignal within the established setpoint(s) 1044, the test is deemed apass (block 1218) and the process 1200 returns to block 1202.

Continuous GFCI Testing

FIG. 13 illustrates an example ground fault detector device 1300electrically connected to a hot (line conductor) 1302 and a neutralconductor 1304 from an electrical source 1308. The example ground faultdetector device 1300 includes a first power diode bank 1310 havingdiodes D₁ and D₂, and a second power diode bank 1312 having diodes D₃and D₄. Each of the first and second power diode banks 1310 and 1312provide power to a GFDC 1314, in which the first power diode bank 1310supplies power to the GFDC 1314 during a negative power cycle, and thesecond power diode bank supplies power to the GFDC 1314 during apositive power cycle. In other words, each of the first power diode bank1310 and the second power diode bank 1312 form a power tap such that thecurrent imbalance is induced for both the positive and negativepower-line cycles, thereby simulating a fault. Current imbalanceconditions may be detected by a sense coil 1316 that, when present andassociated with a fault condition as determined by the GFDC 1314, causea relay coil 1318 to be energized and a power line relay 1320 to open.When the power line relay 1320 is open, electrical energy supplied bythe electrical source 1308 does not reach a load 1322, therebyprotecting the user from potential shock hazards.

In the illustrated example of FIG. 13, the ground fault detector device1300 is performing a self-test continuously. The continuous self-test isparticularly beneficial due to, in part, the continuous nature ofverification that the ground fault detector device 1300 is workingproperly, thereby minimizing shock hazards to end users. In other words,because one or more failures may occur in-between manual, scheduled,and/or periodic self-test operations of other ground fault detectordevices, there exists a possibility of a shock hazard to an end userbefore a subsequent self-test may operate. Additionally, the continuousself-test of the example ground fault detector device 1300 of FIG. 13 isrelatively fast, such that it may detect one or more fault conditionsduring each half power-line cycle (e.g., every 8 milliseconds in U.S.domestic households).

To illustrate, during a positive power cycle diode D₄ conducts toprovide power to the GFDC 1314. By virtue of diode D₄ being locatedafter the sense coil 1316, the current drawn by the GFDC 1314 is sensedby the sense coil 1316, but the return-path current through diode D₃does not pass through the sense coil 1316. As a result, the sense coil1316 detects a current imbalance (I_(FAULT)) that is equal to the amountof current drawn by the GFDC 1314. Additionally, I_(FAULT) is insinusoidal phase with the power-line cycle (as a real fault would be),and because that current is in phase with the line voltage, zerocrossing information may be utilized for switch triggering (e.g., SCR),if needed. The current drawn by the GFDC 1314, and detected by the sensecoil 1316 may be indicative of a quiescent current. In other words, evenwhen the GFDC 1314 is not actively energizing the relay coil 1318 duringa fault, one or more components (e.g., operational amplifiers, filters,voltage-to-current converters, transistors, diodes, a purposelyimplanted load, etc.) draw some current. Similarly, during the negativepower cycle, diodes D₁ and D₂ conduct in a similar manner to supply theexample GFDC 1314 with power. Despite the continuous detection ofI_(FAULT), the example GFDC 1314 does not cause the power line relay1320 to open unless actual (real) fault conditions are also detected, asdiscussed in further detail below.

Continuous detection of I_(FAULT) may indicate proper operation of theexample ground fault detector device 1300 if the value of I_(FAULT) isequal to an expected current draw of the GFDC 1314. In particular,continuous detection of a known I_(FAULT) magnitude indicates that thesense coil 1316 and/or the GFDC 1314 is working properly, therebyproviding the user with protection against fault conditions. However, inthe event of a fault condition (i.e., a real fault caused by an NG faultand/or an LG fault), such as a current imbalance detected by the sensecoil 1316 in excess of a value associated with current draw of theexample GFDC 1314, the power line relay 1320 will trip to protect theuser from the detected fault condition. FIG. 14A illustrates an examplefault separation circuit 1400 within the GFDC 1314 to determine whenI_(FAULT) is related to the continuous self test condition, or whenI_(FAULT) is related to an actual fault condition. The sense coil 1316output value 1402 is separated and provided to a self-test detector 1404and a subtractor 1406. The subtractor subtracts the known fault signalfrom the output value 1402 of the sense coil 1316 and provides thesubtracted signal to a normal-fault detector 1408 (e.g., a comparator).

In operation, the self-test detector 1404 determines whether the outputvalue 1402 of the sense coil 1316 includes the expected quiescentcurrent of the GFDC 1314. Such expected quiescent current may beprovided to the self-test detector 1404 via a threshold 1410. If theoutput value 1402 fails to exceed the threshold 1410, then the self-testdetector 1404 indicates a fault condition to cause the power line relay1320 to open. However, if the threshold 1410 value is at least met orexceeded, the self-test detector 1404 does not send a fault signal tocause the power line relay 1320 to open. Additionally, the normal-faultdetector 1408 determines whether the output value 1402 includes anycurrent imbalance that exceeds the expected quiescent current of theGFDC 1314. In the event that the output value 1402, after the expectedquiescent current signal is subtracted by the subtractor 1406, includesa value in excess of the threshold 1410, then the normal-fault detector1408 provides a fault condition signal to cause the power line relay1320 to open.

Self test verification and fault detection may also be employed by wayof the example fault separation circuit 1450 shown in FIG. 14B, whichmay be realized within the example GFDC 1314. In the illustrated exampleof FIG. 14B, the sense coil 1316 output value 1452 is provided to aself-test detector 1454 (e.g., a magnitude comparator) and anormal-fault detector 1458 (e.g., a magnitude comparator). In operation,the example self-test detector 1454 compares the output value 1452 witha first threshold 1460. The first threshold 1460 may be set to a valuethat, when exceeded by the output value 1452, is indicative of anexpected I_(FAULT) value associated with the quiescent current of theGFDC 1314. In other words, when the output value 1452 exceeds the firstthreshold 1460, no error signal is provided by the self-test detector1454 to open the power line relay 1320. On the other hand, if the outputvalue 1452 fails to exceed the first threshold 1460, then the self-testdetector 1454 provides an error signal to indicate that self test hasfailed, thereby opening the power line relay 1320.

In the event that the self-test detector 1454 determines that the outputvalue 1452 exceeds the first threshold 1460, a fault may still exist. Todetermine whether a fault exists when the output value 1452 exceeds thefirst threshold 1460, the normal-fault detector 1458 compares the outputvalue 1452 against a second threshold 1462. The output value 1452 is asum of both the self-test current imbalance and a real fault currentimbalance, both of which are in-phase. As the two currents are in-phase,any real fault detected will result in an output value magnitudeincrease, which may be compared against the second threshold 1462. Forexample, if the signal magnitude is between the threshold 1460 and 1462,the operation is safe because the simulated fault is detected, but themagnitude is not high enough to indicate that a real fault is present.An error signal is provided by the example normal-fault detector 1458 ifthe output value 1452 exceeds the second threshold 1462, thereby causingthe power line relay 1320 to open.

FIG. 15 illustrates a flowchart representative of an example process1500 that may be executed to implement the example ground fault detectordevice 1300, and the example circuits 1400, and 1450 of FIGS. 13, 14A,and 14B, respectively. The example process 1500 of FIG. 15 begins with,for example, receipt of one or more current imbalance signal(s) from thesense coil 1316 of the ground fault detector device 1300 (block 1502).As described above, because of the diode arrangement for the first powerdiode bank 1310 and the second power diode bank 1312, fault current(s)(I_(FAULT)) detected by the sense coil 1316 associated with either thequiescent current or a real fault will always be in-phase. The receivedcurrent signal (I_(FAULT)) represents a signal that may include only thequiescent current of the example GFDC 1314, a signal that may includeboth the quiescent current and a real fault current, or a signal thatmay include neither the quiescent current nor a real fault current.

In the illustrated example process 1500, the received current signal(I_(FAULT)) is compared with a threshold value associated with anexpected quiescent current (block 1504). As described above, theself-test detector 1404 of FIG. 14A or the self-test detector 1454 ofFIG. 14B may compare the received output value 1402 or 1452 with athreshold 1410 or 1460. If the received output value 1402 or 1452exceeds the threshold 1410 or 1460, then the self test is operatingproperly, and no error signal is provided by the self-test detector(s)1404 or 1454. On the other hand, an absence of the quiescent current isdeemed a failure of the self test, which results in an error signal sentby the self-test detector(s) 1404 or 1454 to open the power line relay1320 and/or signal switch logic (block 1506). However, despite adetermination that there is no error signal by virtue of detecting atleast a minimum current imbalance related to the quiescent current ofthe example GFDC 1314, it is possible that a real fault exists.

To determine whether a real fault exists, the received output value 1402may be provided to the subtractor 1406, as shown in FIG. 14A. Theexample subtractor 1406 subtracts the expected quiescent current signalfrom the received output value 1402 and provides the result to thenormal-fault detector 1408. If the normal-fault detector 1408 determinesthat the result is less than the applied threshold 1410, then no faulthas occurred and no error signal is transmitted by the normal-faultdetector 1408 (block 1508). However, if the normal-fault detector 1408determines that the result is more than the applied threshold 1410, thena fault has occurred because the detected current imbalance exceeds thatof the expected quiescent current drain of the GFDC 1314 (block 1508)plus the minimum fault current. To that end, the normal-fault detector1408 provides an error signal to open the power line relay 1320 (block1506).

Determining that a real fault exists may also be achieved by way of theexample normal-fault detector 1458 of FIG. 14B. The received outputvalue 1452 is compared against the second threshold 1462 (block 1508).If the received output value 1452 exceeds the second threshold 1462,then a fault is deemed true (block 1508) and an error signal is sent bythe normal-fault detector 1458 to open the power line relay 1320 (block1506).

Although certain example methods, apparatus, and articles of manufacturehave been described herein, the scope of coverage of this patent is notlimited thereto. On the contrary, this patent covers all methods,apparatus and articles of manufacture fairly falling within the scope ofthe appended claims either literally or under the doctrine ofequivalents.

1. A ground fault detection device comprising: a sense coil comprising a secondary winding surrounding a line conductor and a neutral conductor, the line conductor and the neutral conductor forming a primary winding; a non-resonant circuit voltage oscillator generating an oscillation signal independent of and connected in series with the secondary winding to drive an oscillation component; and at least one of a detector or an amplifier coupled to the secondary winding to determine a magnitude of at least one of a neutral-to-ground fault or a line-to-ground fault and wherein the at least one detector or amplifier monitors a current of the secondary winding to detect the neutral-to-ground fault, the at least one detector or amplifier comprising a current detector, detecting a current increase, the current increase indicative of a neutral-to-ground fault, wherein the current detector comprises a virtual inductor, an impedance of the virtual inductor being proportional to an impedance of magnetizing inductance of the sense coil, whereby current in both inductances is proportional and the proportion is frequency independent, to provide an accurate fault measurement.
 2. A ground fault detection device as defined in claim 1, wherein the at least one detector or amplifier comprising a current to voltage detector.
 3. A ground fault detection device as defined in claim 1, wherein the detector separates the oscillation component from an induced current of the secondary winding to distinguish between the neutral-to-ground fault or the line-to-ground fault.
 4. A ground fault detection device as defined in claim 3, wherein the detector comprises a line-to-ground detector to detect a current difference of the line conductor and the neutral conductor at a power-line frequency.
 5. A ground fault detection device as defined in claim 1, wherein the at least one detector comprises a line-to-ground fault detector comprising a low pass filter.
 6. A ground fault detection device as defined in claim 5, wherein the low pass filter blocks the oscillation component of the oscillator.
 7. A method to detect a ground fault comprising: driving a secondary winding of a sense coil in series with a non-resonant circuit oscillator, the sense coil comprising a primary winding formed by a line conductor and a neutral conductor, the secondary winding driven at an oscillation frequency generated by the oscillator independent of the secondary winding; detecting a current signal of the secondary winding wherein a current detector comprises a virtual inductor, an impedance of the virtual inductor being proportional to an impedance of magnetizing inductance of the sense coil, whereby current in both inductances is proportional and the proportion is frequency independent, to provide an accurate fault measurement; comparing the detected current signal with a threshold to determine a neutral-to-ground fault; filtering the detected current signal with a low pass filter; and determining a magnitude of the filtered current signal to detect a line-to-ground fault.
 8. A method as defined in claim 7, wherein filtering the detected current signal comprises filtering the oscillation frequency generated by the oscillator.
 9. A method as defined in claim 8, further comprising comparing the filtered current signal to a threshold to detect a differential signal between the line conductor and the neutral conductor.
 10. A method as defined in claim 9, further comprising activating a switch driver when the differential signal is detected, the differential signal indicative of a line-to-ground fault.
 11. A method as defined in claim 10, wherein the activated switch driver causes at least one of illuminating a fault indicator or opening a relay to disconnect the line conductor and the neutral conductor from a load.
 12. A method as defined in claim 7, further comprising activating a fault indicator signal when the detected fault causes the detected current signal to exceed the threshold.
 13. A method as defined in claim 12, wherein the activated fault indicator signal causes at least one of illuminating a light emitting device or opening a relay to disconnect the line conductor and the neutral conductor.
 14. An article of manufacture storing machine accessible instructions which, when executed, cause a machine to: drive a secondary winding of a sense coil in series with a non-resonant circuit oscillator, the sense coil comprising a primary winding formed by a line conductor and a neutral conductor, the secondary winding driven at an oscillation frequency by the oscillator; detect a current signal of the secondary winding wherein a current detector comprises a virtual inductor, an impedance of the virtual inductor being proportional to an impedance of magnetizing inductance of the sense coil, whereby current in both inductances is proportional and the proportion is frequency independent, to provide an accurate fault measurement; compare the detected current signal with a threshold to determine a neutral-to-ground fault; filter the detected current signal with a low pass filter; and determine a magnitude of the filtered current signal to detect a line-to-ground fault.
 15. An article of manufacture as defined in claim 14, wherein the machine accessible instructions, when executed, cause the machine to filter the oscillation frequency generated by the oscillator.
 16. An article of manufacture as defined in claim 15, wherein the machine accessible instructions, when executed, cause the machine to detect a differential signal between the line conductor and the neutral conductor.
 17. An article of manufacture as defined in claim 16, wherein the machine accessible instructions, when executed, cause the machine to activate a switch driver when the differential signal is detected, the differential signal indicative of a line-to-ground fault.
 18. An article of manufacture as defined in claim 17, wherein the machine accessible instructions, when executed, cause the machine to at least one of illuminate a fault indicator or open a relay to disconnect the line conductor and the neutral conductor from a load.
 19. An article of manufacture as defined in claim 14, wherein the machine accessible instructions, when executed, cause the machine to activate a fault indicator signal when the detected fault causes the detected current signal to exceed the threshold. 